Climate Change and Marine Communities 2: What is climate change?

This is the second installment of my serialization of a new book chapter on “Climate Change and Marine Communities” written with Chris Harley and Mike Burrows. It is for a new book “Marine Community Ecology and Conservation” that I’m co-editing with Mark Bertness, Brian Silliman, and Jay Stachowicz. The book is more or less a followup to the best-selling 2001 edition (which is out of print and worth $100 used and $500 new at Amazon!). We asked our authors to tell us what has happened over the last 10 years in their assigned subfield. The chapters are amazing. And I am truly blown away by how much we’ve discovered since the publication of the first edition! Many fields have been revolutionized and many-a-paradigm has been overturned. Cool stuff.

What is climate change?

When most people think of climate change, they think about the greenhouse effect and global warming. The greenhouse effect is caused by gases that heat the atmosphere by “trapping” infrared radiation that would otherwise escape into space. Roughly half of the solar radiation that reaches the atmosphere is reflected back to space or absorbed by clouds, gases, and particles like soot pollution. The other half reaches the earth’s surface and is used in photosynthesis, melts ice and evaporates water, and warms the land and lower atmosphere. This heating emits infrared radiation, some of which is absorbed by greenhouse gas molecules and re-radiated back towards the earth’s surface. This further warms the land and atmosphere.

Although the greenhouse effect is essential to life on earth (without it the surface temperature would be roughly -18º C), human activities have intensified this natural process by increasing the concentration of greenhouse gases in the atmosphere. The two primary gases causing anthropogenic climate change are carbon dioxide (CO2) and methane. Other natural and anthropogenic greenhouse gases include water vapor, nitrous oxide, ozone, and chlorofluoracarbons (CFCs).

CO2 is a less potent greenhouse gas than methane (on a per molecule basis), but the concentration of the former is more than 200 times greater (as of February 2013, the concentration of CO2 was 397 ppm, compared to 1.8 ppm for methane). Because increased CO2 concentration accounts for nearly two thirds of anthropogenic warming, it is considered the most important greenhouse gas in terms of emissions mitigation (and catastrophe avoidance).

Atmospheric CO2 concentration increased by 1.9 ppm per year between 2000 and 2008 (Le Quéré et al. 2012). This rate increased in each of the last four decades, e.g., up from 1.5 ppm per year during the 1990s. The more recent increased emissions rate is primarily due to economic growth in China and other developing nations and the global shift towards coal as an energy source. CO2 concentration is expected to double relative to the preindustrial baseline of 278 ppm during the latter half of the 21st century (IPCC 2007). The resulting global average land surface warming (called the equilibrium climate sensitivity) is “likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C” (quote from IPCC 2007, also see Knutti and Hegerl 2008). The uncertainty around climate sensitivity is due to potential feedbacks in the Earth’s climate system, some of which are not well understood.

Figure 1. Some important abiotic changes to the oceans caused by greenhouse gas emissions. Redrawn from (Harley et al. 2006)